Cosmogenic ^3He in igneous and fossil tooth enamel fluorapatite

Abstract

Igneous fluorapatite samples from a suite of six granitic rocks from the Transantarctic Mountains have high ^3He concentrations (to 5×10^9 atoms g^(−1)) and high ^3He/^4He ratios (to 9×10^(−7)). These values are far higher than those found in several hundred igneous apatites from elsewhere around the world and are higher than can be attributed to nuclear reactions on ^6Li. This ^3He is almost certainly derived from cosmic ray reactions in rocks with high exposure ages at high latitude and elevation. Several samples of fossil tooth enamel fluorapatite from the Turkana Basin of Kenya are similarly rich in ^3He, with up to 1×10^7 atoms ^3He g^(−1) and ^3He/^4He ratios up to 4×10^(−6). Again, this ^3He is most logically attributed to cosmic ray reactions. Provided that cosmogenic ^3He, like radiogenic ^4He, is quantitatively retained in fluorapatite under Earth surface conditions, routine ^3He exposure dating of this common phase may be possible. Based on its chemical composition, the ^3He production rate in fluorapatite is about 100 atoms g^(−1) yr^(−1) at sea level and high latitude. Using this rate the apatites from the Transantarctic Mountains have apparent exposure ages of 0.5–6.2 Myr, in agreement with values elsewhere in the range. The fossil tooth enamel samples have apparent exposure ages ranging from a few up to 130 kyr. Such high exposure ages suggest some of these fossils may be lag deposits with a very long residence time at or near the Earth's surface. ^3He exposure ages can provide insights to the depositional and reworking history of enamel-bearing fossils. At present the major limitations to ^3He exposure dating of fluorapatite are purification of sufficient amounts of material and measurement of small amounts of ^3He in the presence of large quantities of ^4He. In addition, further work is necessary to establish the nucleogenic ^3He background in fluorapatite.

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